Full color organic electroluminescent (EL), also known as organic light-emitting devices (OLED), have been demonstrated recently as a new type of flat panel display. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing, and capability for full color flat emission displays. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic EL medium sandwiched between these electrodes to support charge recombination that yields emission of light. An example of an organic EL device is described in commonly assigned U.S. Pat. No. 4,356,429. Other examples have been described by Tang et al. in U.S. Pat. Nos. 4,769,292, 4,885,211, and 5,550,066. However, in a bottom-emitting type of display where light is emitted downward through the substrate, the overall area that can emit light is limited by the presence on the substrate of thin film transistors (TFT's) and other circuitry, which are opaque. In contrast, top-emitting device structures have significantly higher emission areas than bottom-emitting devices.
Therefore, much work has been done to produce OLED devices, which are top- or surface-emitting. This configuration has the potential to improve display performance compared with bottom-emitting OLEDs by: 1) increasing the aperture ratio, therefore allowing the pixel to operate at a lower current density with improved stability; 2) allowing more complex drive circuitry to enable better control of pixel current, leading to enhanced display performance (uniformity, stability); 3) enabling lower mobility materials, e.g., amorphous silicon, to be considered for TFT fabrication; and 4) allowing schemes for increasing the emission out coupling (increased efficiency) that are not available for the bottom-emitting format. Current designs for top-emitting OLEDs often utilize a reflective metallic anode as the bottom electrode and a semi-reflective metallic cathode as the top electrode. These metallic materials contribute to a microcavity effect within the devices, which can be useful for certain applications. Highly transparent materials, e.g., indium-tin-oxide (ITO), can replace the semi-reflective cathodes; however, these materials are less electrically conductive.
There is a continuing need for improved cathode structures.